The analysis of preignition in a boosted gasoline engine is an important issue to improve the engine power and thermal efficiency.One of the convincing mechanisms is an autoignition of oil-gasoline droplets.The oilgasoline droplets formation process that are adhered to the piston top-land in film is simulated by giving the acceleration of the piston motion.Using VOF method,the effects of several physical parameters,such as oil film thickness,fuel dilution rate,and top-land length on the droplet formulation which disperses in a combustion chamber are investigated.As a result,it was found that droplets formation have three steps and the oil film thickness is the most effective factor.Next,the possibility of ignition of the oil droplet inside the cylinder was examined.However,it was found difficult to cause autoignition.Therefore,CaCO3 included in the oil as an additive may be changed to CaO by heating during expansion and exhaust strokes and then,CaO may be converted into CaCO3 again by absorbing CO2 during the intake and compression strokes.As this change is an exothermic reaction,the temperature of the particle increases determined by CO2 partial pressure.The CaO particle may be a cause of preignition.